window.onload = function () {
    var canvas = document.getElementById("canvas");
    console.log(canvas);
    var context = canvas.getContext("2d");
    context.fillStyle = "lightgreen";
    context.fillRect(0, 0, 400, 400);
    for (var i = 0; i < 10; i++) {
        context.beginPath();
        context.arc(i * 25, i * 25, i * 10, 0, Math.PI * 2, true);
        context.closePath();
        context.fillStyle = "rgb(100,100,100,0.1)";
        context.fill();
    }
}